Figure 1 - uploaded by David James Larner
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The normalized low frequency real characteristic acoustic impedance of the thicker polyester sample. S and D denote a single or double thickness material respectively. The last four predictions show the Equation 10 method, where the two numbers represent the air cavity depths (mm) of the dashed and undashed series respectively.
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Modeling the complex characteristic acoustic impedance and complex wavenumber of porous materials allows the prediction of the complex specific acoustic impedance of a system consisting of porous absorbers and air cavities in front of a rigid surface. By using the transfer matrix method, the complex characteristic acoustic impedance and complex wav...
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... Figure 1, it can be seen that with the seven prediction methods, there was very good agreement over the low frequency range. This shows that so far, there is no failure in the prediction of the complex characteristic acoustic impedance for this material; however, it does break down with the other ...
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Citations
... Smith and Parrott [1] proposed the two-thickness method, while Dunn and Davern [2] measured surface impedance of a single and a double thickness of the same material. Utsuno et al. [3] developed the two-cavity method, and Larner and Davy [4] improved upon it. Iwase et al. [5] presented the three-microphone method, and Song and Bolton [6] introduced the fourmicrophone transfer-matrix approach. ...
... Larner and Davy [4] improved the two-cavity method of Utsuno et al. by replacing one of the air depths with zero depth (rigidly back plate). In Equation 7 and Equation 8, the specific acoustic impedance of air 1 ′ tends to infinity when the depth of the dashed rigidly back air cavity tends to zero (Equation 9). ...
This study presents several methods to determine the characteristic impedance and the complex wave number of porous materials. These methods include the two-cavity method by Utsuno et al., the two-thickness method by Dunn and Davern, the four-microphone transfer-matrix method by Song and Bolton, the empirical method by Miki and theoretical calculation by the inverse method of Johnson-Champoux-Allard (JCA) model. Glass wool and polyester materials are used to compare different approaches. The results indicate a clear correlation among all methods. The theoretical calculation based on the inverse method of the JCA model can predict well without direct measurement of the non-acoustical parameters. However, there are disadvantages to several methods including sharp peaks in some frequency ranges for the two-cavity method, data fluctuation at low frequency for the four-microphone method, and inaccuracy in predicting the characteristic impedance of thick materials at middle and high frequencies for the two-thickness method. The results imply that the inverse method of the JCA model is a reliable method to predict the characteristic impedance and the complex wave number with comparable accuracy to traditional experimental methods.
... Table 1 describes the 10 configurations explored in this study (numbered henceforth as "C1", "C2", etc.), where C1 to C5 are without acoustic foam and C6 to C10 include acoustic foam applied "at the lips". The acoustic foam with a density of ~26 kg.m −3 was introduced as a hollow cylinder with a length of 2 cm in C6 to C8, whereas it was used to fill the space between the transducers and the flange (~4 cm × 4 cm × 2 cm) in C9 and C10 (further acoustic properties of foam can be found at [16]). Open-celled acoustic foam can influence the measurement response of the system [17] and configuration by improving the acoustic coupling between the transducers and waveguide cavity at low frequencies by raising the ZRad at low frequencies. ...
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... In these equations, the dash and undashed symbols imply different air gap depths, k is the wave number in air, q 0 c, which is the product of the ambient air density and the speed of sound in air, is the characteristic impedance of air, and D is the depth of the air gap. Larner and Davy improved Utsuno's method as shown below [15]. The change is that the depth of the dashed rigidly backed air cavity is set to zero and thus its specific acoustic impedance Z 0 1 is 1. ...
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... Achieving a pleasing environment can be obtained by using various techniques that employ different materials. One such technique for reducing noise pollution is to improve sound absorption efficiency by increasing damping capacity and optimizing pore structure of materials [6][7][8][9][10][11]. ...
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A method is proposed for calculation and optimization of sound absorbing performance at low frequencies by choosing an optimal combination of multilayer porous materials, according to their characteristic impedance and propagation constant as two controlling material properties. First, a two-microphone two-load transfer function method according to ISO 10534-2 has been used to obtain the characteristic impedance and propagation constant, where geometrically asymmetrical or multilayer materials can be handled by an equivalent complex characteristic impedance. Second, this material characterization is verified by a direct measurement for various multilayer materials mounted with a hard termination. Last, optimization is performed to obtain the best possible low-frequency sound absorbing performance by comparing combinations of various lengths and characteristic impedances of multilayer materials with a given overall thickness. One advantage of using this method is when a best possible sound absorber needs to be constructed given a collection of realistic materials, with which a suitable theoretical model is yet to be associated, let alone their unknown parameters such as porosity, airflow resistivity and characteristic lengths. This method can be applied in designing preferable sound absorbers, and materials as well if further explored.
... This measurement is performed in both a low frequency and a high frequency two-microphone impedance tube 19 (100 and 29 mm diameters respectively), so a wide range of frequencies is measured (172-5936 Hz). This was performed by gluing the material to a metal mount, so the thin porous material is free standing, slightly less than one-quarter of the wavelength of the maximum frequency away from the rigid backing (this is to prevent any air cavity resonance appearing in the measurement 20 ). The complex specific acoustic impedance of the air cavity used in the measurement is then subtracted from the measurements, so only the complex specific acoustic impedance of the material is added to the theoretical impedance of the air cavity behind the system whose sound absorption is being calculated. ...
This paper studies the diffuse field sound absorption coefficient of a system consisting of a rigid perforated panel with a thin porous woven/matted material glued to its back, which is placed in front of an air cavity with a rigid backing. To cut the cost of trial and error diffuse field sound absorption coefficient measurements, a prediction method was developed. Measurements were made in a two-microphone impedance tube of the complex specific acoustic impedances of the unperforated rigid panel materials and of the thin porous materials in front of a rigidly terminated air cavity. These values were used in the transfer matrix method to predict the complex specific acoustic impedances of the perforated panels systems as a function of the angle of incidence of the sound. These calculations assumed the systems to have infinite or finite lateral extent. The measured diffuse field sound absorption coefficient values usually lay between the infinite and finite predictions. The most important variables are the perforation factor of the panel, the acoustic resistance of the thin porous material and the cavity depth.
This thesis is to document the development of a proposed perforated panel system impedance model, and use this model to improve and create higher performing perforated panels currently created by Atkar, a company who specialises in MDF (Medium Density Fibreboard) perforated panels.
There are several established models published for predicting the specific acoustic impedance of perforated panels systems, especially when backed with and without a bulk porous absorber in the subsequent air cavity behind the perforated panel, or a thin resistive textile glued to the panel. However; these models, based on the equivalent electrical circuit approach (EECA), tend to under-predict the diffuse field sound absorption coefficient at high frequencies (3 kHz and above). Other issues include the lack of taking into the account the impedance of the panel material itself, as well as less modern theory based on the addition of introducing a thin resistive textile into the system. The proposed model in the thesis is to use the transfer matrix method to calculate the specific acoustic impedance of each layer, which is used to improve the high frequency diffuse field absorption coefficient, as well as include a measured panel material impedance, which is placed in parallel with the specific acoustic impedance of the perforations of the panel; and including the measured specific acoustic impedance of a thin resistive textile in front of an air cavity, where the air cavity impedance is theoretically removed from measurements, leaving the impedance of the textile.
This model was then used to improve the current Atkar perforated panel systems, which include the use of a thin resistive textile behind a perforated panel. It was found the currently used resistive textile is too resistive when placed behind a perforated panel, because its impedance is divided by the open area perforation ratio of the perforated panel. Because of this high resistance, introduction of a bulk porous material into the air cavity improved the diffuse field sound absorption coefficient minimally. By implementing an appropriate flow resistance textile, matched to the desired open area perforation ratio of the perforated panel, the diffuse field sound absorption in the low to mid frequencies improved, as well as further improving the system by reintroducing a bulk porous absorber into the air cavity, as the less resistive textile allows more sound energy to penetrate the bulk porous absorber.
Using the theory of the previous model, multiple layered perforated panel systems were theoretically modelled, which include the use of multiple open area perforation ratio panels together in the one system, as well as the introduction of multiple resistive textiles, bulk porous absorbers in between the panels, and the introduction of a micro-perforated panel facing, which is a first for Atkar. Some of these higher performing panel systems were then constructed and tested in a reverberation room, where very high values of NRC were achieved.
Air cavity manipulation was also used to improve the low frequency diffuse field sound absorption coefficient, by creating locally reacting cavities, as well as using impedance transformers such as horns. While in theory, these systems can be very beneficial in improving the low frequency sound absorption coefficient, especially with a combination of horns in different length air cavities, these systems are not yet cost effective to use on a large scale. The use of active noise control was also modelled in conjunction with a perforated panel and resistive textile, which resulted in little or no improvement compared to the improved and current perforated panels systems.
